Magnetically controllable plasma flame generator



Oct. 11, 1966 SACHIQ TAKE] ETAL 3,278,796

MAGNETICALLY GONTROLLABLE PLASMA FLAME GENERATOR Filed June 10, 1965United States Patent 3,278,796 MAGNETICALLY CONTROLLABLE PLASMA FLAMEGENERATOR Sachio Takei, Hachioji-shi, and Manabu Yamamoto, Odawara-shi,Japan, assignors to Kahushiki Kaisha Hitachi Seisakusho, Tokyo-to,Japan, a joint-stock company of Japan Filed June 10, 1963, Ser. No.286,605 2 Claims. (Cl. 315-111) This invention relates to a new andoriginal technique providing a controllable plasma flame generator.

In general, since a plasma flame generator requires a high-powerdischarge of an electric current of a number of tens of amperes or moreand, in terms of electric power, of a number of kilo-volt-amperes ormore, it is diflicult to maintain its discharge in a stable state over along period of time or to achieve fine adjustments of conditions such asthe plasma flame length and temperature.

When a plasma flame of such a generator in operation is examined bymeans of high-speed photography, it is observed that the length andthickness of the ejected flame fluctuate with time, and it can bereadily confirmed that, in accordance with these fluctuations, theintensity of light emitted from the flame also fluctuates. Thesefluctuations are, in all cases, of the order of approximately 10 percentor less. Accordingly, depending on the application, such fluctuations donot present a problem. However, in cases wherein the temperature andintensity of emitted light of the plasma flame is required to be stablewith respect to time, as in the case of application of the flame as alight source for scientific instruments, or in cases such as that of itsapplication as the working fluid in a supersonic wind tunnel, whereinstability of the velocity of the gas flow is required, there is a greatprobability of the above-said fluctuations becoming a problem becausethe stability of the plasma flame, in the operation of these instrumentsand apparatuses, becomes a determinating factor influencing theirperformance.

Among the factors adversely affecting the stability of a plasma flame,the following three factors are the most important.

(1) Migration of electrode foot points of the arc.

(2) Fluctuation of current.

(3) Fluctuation of gas flow rate.

Herein the electrode foot points of the arc designate the points wherethe arc connects to the electrodes (anode and cathode). That is, the arccurrent is supplied through these foot points.

It is an object of the present invention to prevent the occurrence ofthe above-stated migration of electrode foot points of the arc.

More specifically, the present invention contemplates providing acontrollable plasma flame generator by a new and original techniquewhich has been discovered and developed as a result of intensiveexperimental research.

The nature, principles, and details of the invention will be bestunderstood by reference to the following description taken inconjunction with the accompanying drawing, which is a perspective view,partly in vertical section and partly in diagrammatical form, showing apreferred embodiment of the plasma flame generator according to theinvention.

Referring to the drawing, the embodiment shown therein comprises,essentially, a thoriated tungsten cathode 2, a water-cooled copper anode1 which has a plasma flame ejection nozzle 3 formed centrally therein,and means, to be described in greater detail hereinafter, to apply amagnetic field 4 across the said nozzle 3.

Application of a discharge voltage to the anode 1 and cathode 2 causes adischarge current 5 to flow between 3,278,796 Patented Oct. 11, 1966 thetip of cathode 2 as the cathode foot point of the arc and one point onthe anode 1 as the anode foot point. A gas'flow 7 is supplied to flowupwardly past the sides of the cathode 2, past the discharge are, andthrough the nozzle 3, whereby a plasma flame 8 is ejected upwardly fromthe exit of nozzle 3.

The electromagnetic force created by the mutual interaction of theaforesaid magnetic field 4 and the discharge current 5 causes the arcanode foot 6, which, otherwise, would readily migrate, to be heldconstantly at a definite, fixed point. That is, any tendency of the arcanode foot 6 to advance from the said point toward the outside, that is,toward the exit of the nozzle 3, is pre- Vented by the resultingincrease in the electromagnetic force imparted to the discharge arc; andany tendency of the anode foot 6 to retract inwardly from the said pointis prevented by the sweeping action of the gas flow 7. In this manner,it is possible to fix the position of the electrode foot point. In thiscase,'it has also been found through experiments conducted by thepresent inventors that the length and emitted light intensity of theejected plasma flame 8 readily vary in accordance with the in tensity ofthe magnetic field 4. The reason for this is that one portion of theleakage flux intersects the discharge current 5 and has a tendency topress this discharge current against one side of the inner Wall surfaceof the nozzle 3, and the degree of this tendency varies in accordancewith the intensity of the magnetic field.

The present invention, in its principal aspect, makes possible controlof the ejected plasma flame through utilization of the above-describedcharacteristic.

The embodiment shown in the drawing is that of a plasma flame generatorin which the plasma flame is so controlled as to cause its emitted lightintensity to be constant. The afore-mentioned magnetic field 4 iscreated by magnetic field coils 11 and 12 provided with suitably adaptedand disposed cores N and S. These magnetic field coils 11 and 12 arecontrolled by current supplied by a control circuit consisting of aphotoelectric transducer 9 for measuring the emitted light intensity ofthe plasma flame 8 and an amplifier 10 for converting the output of thesaid photoelectric transducer 9 to the desired current value.

By effecting negative feedback of the fluctuation of the emitted lightintensity of the plasma flame to the controlling magnetic field 4 bymeans of the apparatus arrangement described above, it is possible toreduce the fluctuation of the emitted light intensity. Such reduction offluctuation is not limited merely to the emitted light intensity, itbeing possible, by effecting negative feedback of other fluctuationssuch as those of ripples in the power source and the gas flow rate, tocause such fluctuations to be cancelled.

Since a magnetic field of extremely low intensity suffices for control,the ejected plasma flame is not caused to assume an extremelynon-symmetric configuration. If the symmetry of this configuration isparticularly important, the symmetry of the configuration of the ejectedflame can be maintained by causing the position of the electrode footpoint of the arc to vary in a regulated manner by means of aperiodically varying magnetic field, such as an alternating magneticfield or a rotating magnetic field.

For these problems and the problem of the magnetic field intensity, theoptimum point must be suitably determined in accordance with theoperational conditions and use of the plasma jet.

Although the foregoing description has been presented with respect tothe case of automatic control by feeding back the emitted lightintensity, it is also possible to control the state of generation of theplasma flame by manual control or in accordance to a predeterminedcontrol program. It is a unique advantage of the present invention that,in all cases, fine adjustment is possible with an extremely low power.Accordingly, the present invention is particularly effective as meansfor control of the state of generation of the plasma flame or forstabilization thereof.

It is to be observed, furthermore, that the direct-ion of the magneticfield need not always be such as to cross perpendicularly relative tothe discharge current direction. It will be apparent that the essentialrequirement is that the construction of the generator be such as tocause the two directions to cross each other.

While the invention has been described with a particular embodiment, itwill be understood, of course, that modifications, changes and the likemay be made therein without departing from its scope.

What is claimed is:

1. A controllable plasma flame generator which comprises: a rod-shapedfirst electrode; a second electrode surrounding said first electrode andhaving a nozzle; means for creating an arc discharge between the saidtwo electrodes; means for introducing a gas current through thedischarge path of this are so that it may be jetted out of the saidnozzle; means for impressing on the arc discharge path between said twoelectrodes a magnetic field which intersects therewith; and means forautomatically varying the strength of the said magnetic field inresponse to variations in intensity of luminance of the jet plasmaflame; whereby the conditions of are dis-' charge can be controlledoptionally and in a stable manner.

2. A controllable plasma flame generator which comprises: a rod-shapedfirst electrode; a second electrode surrounding said first electrode andhaving a nozzle; means for creating an arc discharge between the saidtwo electrodes; means for introducing a gas current through thedischarge path of this are so that it may be jetted out of the saidnozzle; means for impressing on the arc discharge path between the saidtwo electrodes a periodically varying magnetic field which intersectstherewith; and means for automatically varying the strength of the saidmagnetic field in response to variations in intensity of luminance ofthe jet plasma flame; whereby the conditions of arc discharge can becontrolled optionally and in a stable manner.

References Cited by the Examiner UNITED STATES PATENTS 749,793 1/1904Hewitt 313-161 8/1962 Emmerich 3l51l1 X

1. A CONROLLABLE PLASMA FLAME GENERATOR WHICH COMPRISES: A ROD-SHAPEDFIRST ELECTRODE; A SECOND ELECTRODE SURROUNDING SAID FIRST ELECTRODE ANDHAVING A NOZZLE; MEANS FOR CREATING AN ARC DISCHARGE BETWEEN THE SAIDTWO ELECTRODES; MEANS FOR INTRODUCING A GAS CURRENT THROUGH THEDISCHARGE PATH OF THIS ARC SO THAT IT MAY BE JETTED OUT OF THE SAIDNOZZLE; MEANS FOR IMPRESSING ON THE ARC DISCHARGE PATH BETWEEN SAID TWOELECTRODES A MAGNETIC FIELD WHICH INTERSECTS THEREWITH; AND MEANS FORAUTOMATICALLY VARYING THE STRENGTH OF THE SAID LUMINANCE FIELF INRESPONSE TO VIBRATIONS IN INTENSITY OF LUMINANCE OF THE JET PLASMAFLAME; WHEREBY THE CONDITIONS OF ARC DISCHARGE CAN BE CONTROLLEDOPTIONALLY AND IN A STABLE MANNER.